Greasemonkey is heaven sent if you ever run across sites with useful content but that are persistently "broken" or have usability problems. It lets you write small scripts to modify another sites web pages on the fly as you're visiting them.
A wide range of ready made scripts are available, including scripts to skin del.icio.us, add saved searches to Gmail, show link titles inline for Blogdex, remove assorted filler content from CNN, and much, much more.